One type of compressor for air and other gases is referred to as an oil free compressor. This is a reciprocating compressor in which lubricating oil is not required between a piston head and the adjacent walls of a cylinder in which the piston head is reciprocated. In an oil lubricated compressor, the piston head is sized to only reciprocate in the cylinder. A connecting rod is connected to the piston head with a wrist pin which permits the piston head and connecting rod to rotate relative to each other. During operation of the compressor, oil is splashed or pumped from a sump onto the walls of the cylinder and onto bearing surfaces between the wrist pin and the connecting rod. At least one piston ring seal is provided in an annular groove around the perimeter of the piston to maintain a gas tight seal which prevents leakage of the compressed gas from a compression chamber and prevents most of the lubricating oil from flowing past the piston ring seals to the compression chamber. However, a small amount of lubricating oil may flow past the seal and into the compression chamber and contaminate the compressed gas.
In one common type of oil free compressor, the piston head is formed integrally with the connecting rod so that they do not rotate relative to each other. Since a driven end of the connecting rod is moved about a circle by an eccentric or a crank pin, the piston head will rock or wobble as it is reciprocated in a cylinder. The piston head is relatively thin and sufficient clearance must be provided between the piston head and the cylinder walls to allow the piston head to wobble. Because of the wobble or rocking motion of the reciprocating piston, greater demands are placed on a seal which must extend between the piston head and the cylinder walls. The seal is generally cup shaped and is formed from a resilient, low friction material which will press against and slide along the cylinder walls as the piston head wobbles during reciprocation.
One method used for forming a cup shaped seal on a wobble piston has been to clamp a flat ring or washer shaped piece of seal material to a flat surface on the piston head. The piston head and attached seal ring are forced into a cylinder. As the piston head enters the cylinder, the seal forms a 90.degree. bend next to the cylinder wall to impart a cup shape to the seal. The fibers in the seal at the outside of the bend become highly strained as they are bent 90.degree., weakening the seal. In order to reduce the strain in the seal at the bend, the seal was formed from a softer material than otherwise would be preferred. The softer material is subject to greater wear and consequently has a shorter operating life than may be achieved with a harder seal material.
When the seal is bent into the cup shape, the region of the seal adjacent the bend tends to separate or pull away from the adjacent flat surface on the piston head. Consequently, the seal is not supported adjacent the bend. As the cylinder pressure increases during each cycle of compressor operation, the seal is forced downwardly toward the flat piston head surface, causing the cup bend radius to decrease. The smaller cup radius of the seal increases bending stress on the seal. Since the cylinder pressure varies over each stroke of the piston, the resultant seal bending stress is cyclic. At higher pressures, the unsupported portion of the seal in the region of the bend is forced towards the flat piston head surface, subjecting the seal material to bending fatigue and possible premature fatigue failure. While this problem may occur in a single stage compressor at moderate pressures, it is even more critical in a second stage high pressure cylinder of a two stage oil free compressor. Premature seal failure in the second stage has been an impediment to a successful, commercial two stage oil free wobble piston air compressor.